Moments across up to four pages: stacked in Flow, side by side in Page/Book (BL-0055)

The save sheet gets "+ Previous page" / "+ Next page" (right to left for
rtl in Page/Book) for a picture that runs over several pages. Moments
store a layout (row | column); pages the selected region does not touch
are dropped. Crops, viewer and gallery draw both layouts.

Dialogs now clean up directly instead of waiting for the "close" event,
which a backgrounded page can deliver late (a closed sheet stayed in the
page and M stopped working).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server.js

@@ -850,6 +850,7 @@ const routes = {
     const m = mangaOf(b.m), ch = chapterOf(m, b.c), v = ch.versions.find(x => x.id === b.v) || bad(404, 'Version not found');
     const pages = moments.cleanPages(b.pages, v);
     const moment = { id: crypto.randomBytes(6).toString('hex'), m: m.id, c: ch.id, v: v.id, i: pages, f: pages.map(i => v.pages[i].f),
+      layout: moments.cleanLayout(b.layout),
       region: moments.cleanRegion(b.region), cat: moments.cleanCategory(b.cat), note: moments.cleanNote(b.note), t: Date.now() };
     const list = momentsOf(u);
     if (list.length >= moments.MAX_PER_USER) bad(409, `You already have ${moments.MAX_PER_USER} moments.`);